This invention is generally related to electronic buffer circuits and more particularly to precision buffers which are sometimes called upon to drive a large, capacitive load.
Pecision buffers that feature a closed loop negative feedback amplifier (e.g. a unity gain operational amplifier) are used in a wide range of electronic systems to provide increased drive capability for a precise, predetermined voltage. For instance, in image sensor integrated circuits (ICs), a precise analog signal may need to be distributed to each photocell of the sensor array. Although the capacitance presented by each photocell is relatively small, the tens of thousands of such photocells, as they appear in modern high resolution image sensor ICs, present a significant capacitance to the output of a precision buffer. This causes an output node voltage of the buffer to temporarily sag until the capacitive load has been charged up to the set value by the precision buffer. In some applications, the time required for the precision buffer to return its output node voltage to its set value may be tolerated. However, in certain time critical applications such as resetting the photocells of the sensor array in preparation for capturing a sequence of images, it is desirable that the output node voltage be returned to its set value, following the initial drop, in a relatively short period of time. Although the conventional precision buffer is able to accurately return its output to its predetermined set value, its ability to do so in a very short period of time is limited. A limited solution to this problem may be to use a class AB buffer which allows a relatively fast and accurate recovery of the set value in the presence of a large load. However, class AB buffers present the circuit designer with a complex and time consuming design and manufacturing task due to their complex structure. Moreover, in ICs whose performance may be affected by increased heating in the IC, the use of class AB buffers becomes less desirable due to their continuous power consumption characteristics.
According to an embodiment of the invention, an apparatus is disclosed having a closed loop negative feedback amplifier and a precharge circuit. The amplifier is coupled to continuously drive a node to a predetermined voltage. The precharge circuit is coupled to selectively drive the node at a higher rate than the amplifier.